Individuals globally are invited to help scientists discern which sources warrant rapid follow-up-potential kilonovas-and which are false signals. The inaugural, and thus far sole, kilonova observation occurred on August 18, 2017: a fleeting flash of light resulting from the merger of two neutron stars. This event led to the creation of a stellar mass black hole. Remarkably, gravitational waves were also detected in the milliseconds preceding the merger, marking the first instance where astronomers observed both gravitational waves and electromagnetic radiation from the same phenomenon.

BlackGEM
The light emitted during a kilonova diminishes swiftly, remaining detectable for only a few days. Prompt action is essential, with astronomers directing telescopes toward the sky region where the gravitational wave originates. However, gravitational wave detectors like LIGO and Virgo can pinpoint the location only within an area spanning hundreds of square degrees (for context, the full Moon covers about 0.2 square degrees), much larger than the field-of-view of even the most expansive telescopes.

To enhance location accuracy, astronomers have developed specialized telescopes to rapidly identify the faint optical signals associated with merger events. A notable addition is the sensitive BlackGEM-array of telescopes in Northern Chile. Upon detecting a gravitational wave signal, BlackGEM swiftly surveys the vast sky region identified by the detectors. Comparing these fresh observations with previous ones yields numerous candidate sources. Among these might be a kilonova resulting from the merger of two neutron stars and the emergence of a new black hole.

False sources
"However, as a large area of the sky has to be searched, false, non-astronomical, signals can occasionally slip through our AI-trained filters," explains Steven Bloemen (Radboud University, the Netherlands), project manager of the BlackGEM telescopes.

A frequent source of false signals is light reflecting off communication satellites. "In addition, BlackGEM also detects signals with an astronomical origin, but which are unrelated to the kilonova signal we are looking for, such as near-Earth asteroids," says Peter Jonker (Radboud University, the Netherlands), PI of the citizen science app and co-PI of the Dutch Black Hole Consortium.

Global citizens are encouraged to assist astronomers in distinguishing fake sources from potential candidates for further observation. "Even among these astronomical signals that are not due to the kilonova, there are events related to black holes," says Paul Groot (UTC, Radboud University and PI of BlackGEM).

AI training
Given the vast number of candidate sources, astronomers employ artificial intelligence techniques to determine which sources are noteworthy and which can be disregarded. Steven Bloemen adds: "People are still much better at identifying patterns than our algorithms. By using the app, citizens across the world can help train our AI-algorithms to distinguish between real and false sources and pinpoint the most interesting candidate sources more quickly."

Users who demonstrate proficiency in identifying genuine sources can now initiate follow-up observations with the Las Cumbres Observatory (LCO) network of robotic telescopes. Jonker adds: "The LCO director has kindly agreed to allow citizens to trigger their 0.4m telescopes to conduct follow-up observations directly from the app when the user deems this necessary. This will provide information astronomers can use to determine if one of the real events is a kilonova." Edward Gomez, LCO education director adds: "We are delighted to see LCO being used for this citizen science project, and in different languages, making astronomy more accessible to a wider audience."

Danielle Pieterse, PhD student at Radboud University and involved in BlackGEM and the development of the BlackHoleFinder app: "Potential kilonova signals can come at any time, day or night, and they evolve rapidly so time is of the essence. That's why the BlackGEM data is available in the app around the world only 15 minutes after the telescope has taken the data. The global reach of the app is also crucial - with citizen scientists across the whole world, there will always be someone awake to quickly check the new data."

Each observing night in Chile, the BlackGEM array will uncover new transient sources. App users can also view the telescope live within the app. No data is collected during daylight hours in Chile, so new data will not be available then. The LIGO/Virgo/KAGRA consortium of gravitational wave detectors will be actively "listening" for new gravitational wave signals until June 2025.

The BlackHoleFinder-app, developed by the Dutch company DDQ Pocket Science, is available in the Apple- and Android-app stores in English, Dutch, Spanish, German, Chinese, Bengali, Polish, and Italian.

The app is available in the Apple and Android app stores, and here.

There is also a desktop version available here

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